Certain embodiments of the present invention are directed to integrated circuits. More particularly, some embodiments of the invention provide voltage regulation. Merely by way of example, some embodiments of the invention have been applied to power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system. The power conversion system 100 includes a primary winding 102, a secondary winding 104, an auxiliary winding 114, a power switch 106, a current sensing resistor 108, two rectifying diodes 110 and 116, two capacitors 112 and 118, three resistors 120, 122 and 124, and a system controller 160. For example, the power switch 106 is a bipolar transistor. In another example, the power switch 106 is a field effect transistor (e.g., a metal-oxide-semiconductor field effect transistor).
As shown in FIG. 1, the power conversion system 100 uses a transformer including the primary winding 102 and the secondary winding 104 to isolate a primary side and a secondary side of the power conversion system 100. Information related to an output voltage 126 on the secondary side can be extracted through the auxiliary winding 114 and a feedback signal 154 is generated based on information related to the output voltage 126. The controller 160 receives the feedback signal 154, and generates a drive signal 156 to turn on and off the switch 106 in order to regulate the output voltage 126.
When the power switch 106 is closed (e.g., on), the energy is stored in the transformer including the primary winding 102 and the secondary winding 104. Then, when the power switch 106 is open (e.g., off), the stored energy is released to the output terminal, and the voltage of the auxiliary winding 114 maps the output voltage 126 as follows.
                              V          FB                =                                                            R                2                                                              R                  1                                +                                  R                  2                                                      ×                          V              aux                                =                      k            ×            n            ×                          (                                                V                  O                                +                                  V                  F                                +                                                      I                    O                                    ×                                      R                    eq                                                              )                                                          (                  Equation          ⁢                                          ⁢          1                )            where VFB represents the feedback signal 154, Vaux represents a voltage 158 of the auxiliary winding 114, R1 and R2 represent the resistance values of the resistors 122 and 124 respectively. In addition, k represents a feedback coefficient, n represents a turns ratio of the secondary winding 104 and the auxiliary winding 114, and Req represents a cable resistance 120. Further, VO represents the output voltage 126, IO represents an output current 128, and VF represents a forward voltage of the rectifying diode 110.
A switching period of the switch 106 includes an on-time period during which the switch 106 is closed (e.g., on) and an off-time period during which the switch 106 is open (e.g., off). For example, in a discontinuous conduction mode (DCM), a next switching cycle does not start until a period of time after the completion of a demagnetization process associated with the transformer including the primary winding 102 and the secondary winding 104. In another example, in a continuous conduction mode (CCM), a next switching cycle starts before the completion of the demagnetization process. Thus, the actual length of the demagnetization process before the next switching cycle starts is limited to the off-time period of the switch 106. FIG. 2(A) and FIG. 2(B) are simplified conventional timing diagrams for the power conversion system 100 that operates in the DCM mode and in the CCM mode, respectively.
FIG. 2(A) is a simplified conventional timing diagram for the flyback power conversion system 100 that operates in the discontinuous conduction mode (DCM). The waveform 170 represents the voltage 158 of the auxiliary winding 114 as a function of time, and the waveform 172 represents a secondary current 162 flowing through the secondary winding 104 as a function of time.
Three time periods are shown in FIG. 2(A), including an on-time period Ton, an off-time period Toff and a demagnetization period TDemag. For example, Ton starts at time t0 and ends at time t1, TDemag starts at the time t1 and ends at time t3, and Toff starts at the time t1 and ends at time t4. In another example, t0≦t1≦t2≦t3≦t4.
The controller 160 often implements a sample-and-hold mechanism. When the demagnetization process on the secondary side of the power conversion system 100 is almost completed (e.g., at t3), the secondary current 162 becomes almost zero (e.g., as shown by the waveform 172). The voltage 158 of the auxiliary winding 114 is usually sampled at t2 (e.g., point A). The sampled voltage value is often held until the voltage 158 is sampled again during a next demagnetization period. Through a negative feedback loop, the sampled voltage value can become equal to a reference voltage Vref as follows:VFB=Vref  (Equation 2)Thus, the output voltage 126 can be determined as follows:
                              V          O                =                                            V              ref                                      k              ×              n                                -                      V            F                    -                                    I              o                        ×                          R              eq                                                          (                  Equation          ⁢                                          ⁢          3                )            
FIG. 2(B) is a simplified conventional timing diagram for the flyback power conversion system 100 that operates in the continuous conduction mode (CCM). The waveform 202 represents the voltage 158 of the auxiliary winding 114 as a function of time, the waveform 204 represents a secondary current 162 flowing through the secondary winding 104 as a function of time, and the waveform 206 represents a primary current 164 flowing through the primary winding 102 as a function of time.
Three time periods are shown in FIG. 2(B), including an on-time period Ton, an off-time period Toff and a demagnetization period TDemag. For example, Ton starts at time t5 and ends at time t6, TDemag starts at the time t6 and ends at time t8, and Toff starts at the time t6 and ends at the time t8. In another example, t5≦t6≦t7≦t8.
The controller 160 often implements a sample-and-hold mechanism. When the switching cycle related to the secondary side of the power conversion system 100 almost ends (e.g., at t8), the secondary current 162 becomes close to Isec_2 (e.g., as shown by the waveform 204). The voltage 158 of the auxiliary winding 114 is usually sampled at t7 (e.g., point B). The sampled voltage value is often held until the voltage 158 is sampled again during a next demagnetization period. Through a negative feedback loop, the sampled voltage value can become equal to a reference voltage Vref as follows:VFB=Vref  (Equation 4)
Thus, the output voltage 126 can be determined as follows:
                              V          O                =                                            V              ref                                      k              ×              n                                -                      V            F                    -                                    I              O                        ×                          R              eq                                                          (                  Equation          ⁢                                          ⁢          5                )            
The system 100 has some disadvantages. For example, parameter variations of certain circuit components may affect the voltage regulation. Hence, it is highly desirable to improve techniques for voltage regulation of power conversion systems.